Decompression Diving
Introduction
By Jon Richards and Steven Winstanley
It's all about Ascent, Ascent, Ascent. BSAC said in the 2007 DOC, that DCI incident where on the increase. In diving today, Nitrox is used from the most basic levels of diving, Trimix enables venture to more remote depths and Rebreather diving has become more common. Diving on mix gasses is the norm and this tends to lead to deeper depths and longer times.
 |
The Incident report for 2007 contains 91 ascent related incidents. The BSAC Incident Report 2007 says:
"Ascent Related Incidents
This category of incidents is now the biggest, having suffered a dramatic rise over the last ten years. As I have stated before, the great majority of these incidents relate to poor buoyancy control and rapid ascents, often resulting in missed decompression stops.
Better training is the key; BCD and drysuit control, and the use of associated equipment, especially delayed SMBs. These are avoidable problems and instructors should make this a priority area for attention. Many DCI cases have their roots in these problems; they have been recorded under the DCI heading, but the causal factors are often the same, so the actual number of abnormal ascents will be significantly higher than shown above."
For decompression incidents the BSAC database contains an analysis of the causal factors associated with the cases for 2007 indicates the following major features:-
- 24 involved diving to deeper than 30m
- 24 involved rapid ascents
- 28 involved repeat diving
- 13 involved missed decompression stops
As reported many times before, poor buoyancy control is at the heart of the majority of these cases. Divers are failing to correctly control their ascent, especially in the critical last 10m zone and ending up with rapid ascents and/or missed decompression stops. Very often the diver is using a drysuit and is unable to prevent a buoyant ascent.
Delayed surface marker buoys (DSMB) have been implicated in a number of incidents. Typical problems are loss of buoyancy control whilst deploying the DSMB, jammed reels, tangled lines and free flowing regulators. These problems can result in a rapid ascent to the surface. The very piece of equipment that is supposed to increase the safety of an ascent is having just the reverse effect.
Clearly much more training and practice is required in the use of this system. If possible the reel should be fastened to the bottom (rock or wreck) during the deployment and it should not be attached to a diver so that, in the last resort, it can simply be abandoned if there is a problem.
The safest ascent will always be with a correctly deployed shotline but this, of course, is not always appropriate or possible.
Layman's Deco Diving
We don't pretend to know about decompression. No-one in the world with any brains will stand up and say they do. We've have read dozens of articles (understood less than half of them but we're getting there), so far, we've passed three decompression courses and bought loads of books. I'll try and sum up what I've been taught in simple terms. If you want to be bored by diagrams and formulae let Jon know as he's going through a really sad phase where this and physiology is all he read about.
It's a lie to say they we only absorb nitrogen under increased atmospheric pressure. We have dissolved nitrogen in our blood right now. If you were transported to the top of Everest in an instant we would get a monumental bend due to the decreased pressure causing the nitrogen to bubble (This is why aircraft cabins are pressurised to 1500-2000m). Stopping bubbles is the key.
Decompression diving
Decompression is a glass ceiling. Any dive where you can't make a slow and safe ascent directly to the surface without stopping is a decompression dive. We're told you can dive to the limits of the PADI Residual Dive Planner, make an 18m per minute ascent and be okay. Rubbish. Everyone is different and people have been bent diving to these limits. In fairness to the agencies they had to come up with limits and the PADI RDP is the best agency RDP on the market bar none. How did they arrive at these limits? RDPs are based on US Navy tables. The US Navy created a set of tables. They then called upon their divers to dive the limits and ascent rates. If more than one percent of the divers got bent they adjusted the tables. Simple and barbaric, but it gave us our tables. Bear in mind that the US Navy divers are all of a similar age and well above average physical fitness. PADI altered the tables to fit the average US diver. We've all seen the average US citizen (40% of US citizens are clinically obese - Muscle and Fitness magazine).
We've all done a planned three metre safety stop for three minutes (otherwise we shouldn't have passed the PADI advance course). In that case we've all done planned decompression regardless of what the agencies tell us.
In everyday terms, decompression diving refers to dives where you exceed the no-stop limits of recreational planners and plan decompression stops and ascent rates accordingly. Or plan them on the hoof when we screw up.
So what does a decompression dive look like?
 |
Pyle Stops
Just a comment on Pyle Stops. This is a techie technique that helps you get into the "exponential" accent to the surface. Pyle stops are calculated as follows.
- Calculate a decompression profile for the dive you wish to do, using whatever software you normally use.
- Take the distance between the bottom portion of the dive (at the time you begin your ascent) and the first "required" decompression stop, and find the midpoint. You can use the ambient pressure midpoint if you want, but for most dives in the "technical" diving range, the linear distance midpoint will be close enough and is easier to calculate. This depth will be your first deep safety stop, and the stop should be about 2-3 minutes in duration.
- Re-calculate the decompression profile by including the deep safety stop in the profile (most software will allow for multi-level profile calculations).
- If the distance between your first deep safety stop and your first "required" stop is greater than 10 Metres, then add a second deep safety stop at the midpoint between the first deep safety stop and the first required stop.
- Repeat as necessary until there is less than 10 Metres between your last deep safety stop and the first required safety stop.
Deco Computer Features
Watch & Mechanical Depth Gauge
Should be a part of everyone's kit. This should be your primary source of decompression information. If you dive with these items regularly, then you will not forget to set the maximum depth needle or adjust the watch bezel when you start to dive. So when things go wrong, you have both your computer AND you know what you have to do on the tables to get back to the surface. You computer should be treated as your back up.
D-Timer
A simple digital depth gauge shows dive time, current depth and maximum depth. When factory-programmed ascent limits are exceeded an alarm sounds. In the UWATEC d-timer it's 10m/min, it's the same with Suunto.
Computers
The difference between a d-timer and a computer is that a computer is based on a decompression algorithm. The algorithms come in many forms and are all proprietary (ZH-L8 etc. I can explain the basics but that's an even longer text). They have one thing in common, they are nearly all based on the same basic tables. Modified Haldane's tables (turn of the century) from Scotland, Buhlmann of Zurich (hence the ZH in the algorithm above) and the US Navy did the same.
All computers measure temperature even if they do not display it and alter no-stop limits and decompression accordingly.
Ascent Rates
Suunto use a fixed ascent rate of 10m/min, except from 6M where is slows down to 3M/min. UWATEC have a variable ascent rate that gets slower the closer you get to the surface. That's why you hear our computers beeping yet we're still within the ascent rates taught on the PADI OW course. The more expensive computers let you set it.
Bubble Models
Bubble models limit the growth of nitrogen bubble to a certain size and number. The computers that include VPIM and RGB bubble models are very expensive and in some cases still experimental.
Nitrox vs Air Another misnomer
Nitrox is any gas containing nitrogen and oxygen. Air is a Nitrox mix. Enriched Air Nitrox (EAN) is what we should all be calling it. Less nitrogen in the mix means longer bottom times and less narcosis. The extra oxygen means shallower Maximum Operating Depth (MOD) due to the fact that oxygen toxicity will now set in sooner.
Air Integrated
Allows you to have the cylinder pressure and breathing rate on the screen. Can be connected via a hose or a wireless transceiver. Doubles the price of the unit.
Computers makes in Winter 2007/8
UWATEC Aladdin Pro Ultra
Nitrox mixes up to 50%. Great screen. Has wet contacts instead of buttons for switching between modes. Infuriating and almost impossible when wet. First time it beeps to tell you decompression stops are required is frightening if like me you didn't bother to RTFM (Read The Flipping Manual). Variable ascent rate. Visual and audible alarms for ascent rate, oxygen toxicity etc. The elastic strap is rubbish. Cut it off and replace with bungee or a couple of knife straps, {your knife shouldn't be tied to your leg anyway ;-) } or make sure you get a lanyard. Datatrack software to download dives is okay. The datamouse used to connect it to the PC is rubbish. Fails more often than works. Software is free. You can source the datamouse bits for about £15 quid. The datamouse is sold for £85. Utter rip-off.
Smart Pro and Smart Com
Smartcom is air integrated connect via a hose. The UWATEC Smart Pro/Com is not a true bubble-model computer. It merely lets you program in conservatism to allow for cold, age, fitness etc. Features as per Aladdin Pro Ultra
Aladdin Sport, Aladdin Pro Basically an air-only version of the above. May come with watch-style strap.
Suunto Vytec
Nitrox, air and d-timer. Gas switching.
It has a smaller screen than Aladdin. Shows temperature and real-time clock. The software is simple easy to use, as with the Aladin the PC interface is a rip off at £80. Ascent rates fixed at 10m/min. Has buttons to switch between modes. Air integrated option that can be bought afterwards and thus you don't have a huge expenditure all at once. It has d-timer mode for Trimix divers.
Gas switching. The Vytec allows you to pre-program a decompression gas. Example. I did a 40m dive for 30-35 min bottom time on air. Jon had 35 mins of stops, and Steven on a Vytec switched his computer to 100% O2 and reduced his stops to 16mins. Git!
The Vytec is a great computer for Nitrox and is possible to programme a travel mix and decompression gas. If you progress to Trimix then the d-timer is adequate.
Vyper
More or less as above but with no gas switching.
Buddy Nexus.
As the Vytec but with two gas switching options, the main benefit is that it will do CCR decompression schedules handy if you own an evolution, (or wanting to go in that direction). The unit is expensive but good we consider a Vytec is better though. Main problem is that if you float past 67M it will give up calculating depth, although you should not be down that deep with this unit.
Opinion Time
I've tried to be factual, there are general mistakes in the above, but we don't have all day to write the article, therefore check around.
If like me you plan to do wreck diving where all the action is at a fixed depth don't bother with a computer. Nothing that a watch, dive table and free dive software off the Internet can't cope with. If you want accuracy of depth (my analogue gauges read 28 & 32m respectively at 30m - both same make), get a d-timer. The UWATEC d-timer is ~£80 and lasts for ten years depending on the amount of dives. It has a basic log book function. This is all I plan to use until I make the financially suicidal decision of trimx/rebreathers. Then I'll get a VR3 programmed with RGBM. They start at ~£700. My d-timer in 28.1m of fresh water (~35 degrees Celsius) reads 28.7m. The Aladdin reads perfect. I can cope with that tolerance. It errs on the side of safety. All computers are calibrated in freshwater and are therefore inaccurate in salt water. Why fresh water? Well, there is various salinity of salt water depending where you are in the world.
If as well as UK wreck-diving you plan a couple of holidays a year dropping into the blue down a reef wall and ascending to different points i.e. multi-level dives then you'll be hard pushed to beat any of the above computers for sheer convenience. Repetitive dives are factored in. Again, nothing a set of tables and a watch can't do but I like planning dives. PADI wheel? Load of rubbish. I can teach you everything the wheel does with the PADI RDP in ten minutes.
If you decide on a computer then definitely go for Nitrox. At the moment I'd recommend the Vytec. I'm willing to bet in a few years (apart from the d-timer) all computers will be sold with nitrox capabilities. Ok, I mean EAN capabilities. I think everyone should do a Nitrox course. The benefits in the 20-40m range (majority of our diving) far outweigh the training and gas costs. If you decide EAN is not for you then get a second hand air computer off the Internet.
Air integrated computers are a waste of money. Go and buy another piece of kit with the money. You should know your air-consumption by now, and these computers don't work with independent twinsets or take any notice of the travel and deco cylinders. If I'm things have hit the fan underwater, I know my air rate will go up. If I am trying to stop, breath, think, act a beeping noise caused my air rate has going up will not help.
I don't see the point of putting all your eggs in one basket. If you have a fancy air integrated computer on your hose and it blows on holiday you've lost your depth gauge, air pressure gauge and timer all in one. Not to mention your air! I can lose any or all of the above on a dive and get out safe. I do dive on a twinset though.
Do not believe anyone in a shop. Anyone who works in a shop and says "I'm, a diver mate - not a salesman" is a liar. Again, do the right thing by asking around and we're happy to help. Beware battery changes, a Mares and Suunto computers comes with user-replaceable battery. Perfect. Aladdin compromises the seal of the computer and charges you ~£60 for the privilege. If it leaks they will probably say you abused it.
Technical Diving Books
Jon's nearly finished Kevin Gurr's Technical Diving from the bottom up. Not bad but not a lot of new stuff. Assumes that the diver is already tech and wants another opinion. Good for those wanting an insight to expedition planning. Presentation, spelling, grammar etc dreadful. First aid stuff not much use. Keeps mentioning Cyanosis but doesn't explain what it is (like this with many terms - no glossary). Recovery position incorrect and CPR breaths to compression ratios wrong. They wouldn't be life-threatening, but I think he's used the values he's used for years. Lot of content but some of the subjects are only mentioned. Either do it properly or omit them. The bit on marine stings etc is rubbish. Section on rebreathers but can be annoying. Keeps saying that rebreather divers incur less deco but doesn't say why. Running a high PO2 does not answer it for me. Will have to read up on rebreather technology next!
Lipmann's Essentials of deeper sport diving is well written. Spends a lot of time comparing tables. Goes into the history of deeper diving a bit. Not many mistakes.
Gilliam's Deep Diving. Probably the best of the bunch. Goes into a lot of history of deep diving. Goes on about himself a bit though. Lots of spelling mistakes and some of the tables have errors. It's either the UPTD or CNS table that is incorrect.
Rod Palmer's introduction to technical diving is mostly about kit and configurations. The book has a little theory and is well presented and easy to follow. Not bad but not as meaty as the others. What content is there you can't fault.
Jon's also just read Jolie Bookspans diving physiology in plain English. Understood more about deco from that book than any of the others. Explains m-values, supersaturation etc. it has great explanations of physiology. The most enjoyable of the lot to read but there is no diving technique involved. I would recommend this to all divers regardless of aspirations.
Steven's panning his way through Bruce Wenkie's Technical Diving in Depth, it's very much for the mathematician as I'm sure I've seen some of the formula's in the Theory of Relativity. So far I would recommend the book, but you will need some time and a piece of paper.
Internet Articles on Decompression Theory
Have a look at these sources of information.
All the Richard Pyle articles (first three here) are easy to digest.
http://www.bishopmuseum.org/research/treks/palautz97/cmd.html
Scary
http://www.iantd.com/rebreather/phys.html
Enjoyable reading - doesn't get too deep.
http://www.bishopmuseum.org/research/treks/palautz97/deepstops.html
The background behind Pyle stops. Andy H sort introduced this on one on the ER course.
http://www.decompression.org/maiken/home.htm
The bubble decompression strategies section is good.
